In animals the seven enzymes involved in de novo fatty acid synthesis are integrated into a novel homodimer structure. Growth of the fatty acyl chain on the multienzyme is normally terminated at 16C atoms by the thioesterase I component of the complex, resulting in the formation of palmitic acid. In mammary glands, the product specificity of the multienzyme is modified by a unique regulatory enzyme, thioesterase II, which releases medium chain acids (C sub 6 - C sub 12) from the multienzyme; these medium chain acids are important constituents of milk fat. The properties of thioesterase I and II are being studied from the aspect of structure, specificity and amino acid sequence around the active site, to establish a basis of their different functions. The controversial issue as to the role of free CoA in the fatty acid synthetase reaction is being investigated to determine whether it is required in the elongation or termination reactions. Finally, approaches are being used in vitro and in vivo to establish the mechanism of hormonal regulation of thioesterase II expression in the developing mammary gland.